Elongated pressure sensor for a pressure transmitter

ABSTRACT

A transmitter in a process control system transmits a pressure over a process control loop. The transmitter includes I/O circuitry, compensation circuitry and an absolute pressure sensor. The I/O circuitry transmits information over the process control loop. Compensation circuitry receives a pressure related signal and responsively controls the I/O circuitry to transmit pressure information on the loop. The absolute pressure sensor includes a cavity which deforms as the sensor deflects in response to an applied pressure. A sensor in the cavity provides the pressure related signal to the compensation circuitry in response to the deformation.

This is a Continuation of application Ser. No. 08/395,778, filed Feb.28, 1995, now U.S. Pat. No. 5,637,802.

BACKGROUND OF THE INVENTION

The present invention relates to the process control industry. Inparticular, the invention relates to a pressure sensor in a pressuretransmitter.

Pressure transmitters in process applications measure pressure of aprocess and responsively communicate the information over a two-wireprocess application loop, for example a 4-20 mA current loop. Pressuresensors in transmitters typically comprise some type of a pressureresponsive structure which has a deflectable diaphragm that moves inresponse to applied pressure. These structures can be used to measureboth absolute and differential pressure. As used herein, a differentialpressure sensor is a sensor which measures a relatively small pressuredifferential (such as that generated across an orifice in a flow tube orbetween two different heights in a fluid filled container) over arelatively wide absolute pressure range. In a typical prior arttransmitter, to measure differential pressure, two different pressuresare applied to opposing sides of the structure causing a relativedeformation in the structure which is measured. Measurement of thedeformation, for example, can be by measuring a change in electricalcapacitance due to movement of capacitor plates carried on the structureor by change in resistance of a resistive strain gauge.

Highly accurate absolute pressure sensors have been desired. However, ithas been difficult to obtain an absolute pressure sensor which candeliver an accurate output over a wide pressure range, from 0.4 psi to4000 psi for example. It would also be desirable to measure differentialpressure with two absolute pressure sensors because this is mechanicallymuch simpler than it is to mechanically couple two pressures to adifferential pressure sensor. Additionally, an over-pressure in a such adifferential pressure sensor can damage the differential pressuresensor.

However, it has been difficult to obtain absolute pressure sensors withsufficient accuracy to allow differential pressures in the 0.4 psi to 40psi range to be measured in a device which must withstand static or linepressure extremes of as much as 4000 psia. For example, 0.01% of 4 psidrequires 0.00001% of 4000 psia (10⁻⁷ or 0.1 ppm).

Typical known pressure sensors used in process applications haveunit-to-unit variations in sensitivity to sensed pressure as well asunit-to-unit variations in undesired responses to extraneous parameterssuch as temperature. This can be a particular problem when the outputsof two absolute or gauge pressure sensors are combined to provide anoutput representing differential pressure or when the sensor is usedover a large pressure range. Additionally, mechanical stress associatedwith mounting the sensor to the pressure transmitter results inrelatively large errors in pressure measurement.

SUMMARY OF THE INVENTION

A pressure transmitter in a process control application for transmittingpressure on a process control loop includes an absolute pressure sensor.The absolute pressure sensor has a cavity therein in which cavity wallsdeform or are placed under stress as the walls respond to appliedpressure. The pressure sensor includes a support structure whichprovides stress isolation. A sensor coupled to the cavity walls providesa pressure related output signal. In one embodiment, the sensor andsupport structure are integral with one another such that there are nojoints between the sensor structure and the support structure. Thematerial and dimensions of the pressure sensor are selected such thatthe pressure related signal is very accurate and may be used over a widepressure range or in pairs as differential pressure sensors. In oneembodiment, the pressure sensor comprises single crystal sapphire in anelongated shape and is adapted for immersion directly in process fluid.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a pressure transmitter.

FIG. 2 is a cut-away perspective view of an insert which carries apressure sensor.

FIG. 3 is a cross-sectional perspective view of a pressure sensor.

FIG. 4 is a graph which illustrates operation of the present invention.

FIG. 4A is a graph of stress attenuation versus L/W for a pressuresensor.

FIG. 5 is a displacement plot of one-quarter of a pressure sensor wherel/T equals 1.0.

FIG. 6 is a cross sectional view of another embodiment of the pressuresensor.

FIG. 7 is a cross-sectional view of a pressure sensor.

FIG. 8 is a cross-sectional view of the pressure sensor of FIG. 7.

FIG. 9 is a top plan view of a top substrate of the pressure sensor ofFIG. 7.

FIG. 10 is a top plan view of a bottom substrate of the pressure sensorof FIG. 7.

FIG. 11 is a schematic diagram of circuitry for measuring capacitance ofa pressure sensor.

FIG. 12 is a cross-sectional view of a sensor body.

FIG. 13 is a bottom plan view of the sensor body of FIG. 12.

FIG. 14 is a cross-sectional view of a sensor body.

FIG. 15 is a bottom plan view of the sensor body of FIG. 14.

FIGS. 16A through 16G show cross-sectional views of various embodimentsof the invention.

FIGS. 17A and 17B show two embodiments of capacitor plates.

FIG. 18 is a graph of bonding temperature as a percent of melting pointtemperature versus bonding strength as a percent of material strength.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows pressure transmitter 50 having transmitter body 52, sensorbody 54 and flange 55. Sensor body 54 includes pressure sensors 70 A and70 B which measure absolute pressure P1 and absolute pressure P2,respectively, of process fluid. Transmitter body 52 includes transmitter(I/O) circuitry 60 which sends information related to pressures P1 andP2 over a two-wire process control loop, such as a 4-20 mA current loop.Circuit board 57 couples sensor circuit board 58 to sensors 70A and 70Band receives electrical signals related to pressures P1 and P2.Circuitry on sensor circuit board 58 digitizes and processes thesesignals, and communicates pressure information to transmitter circuitry60 using data bus 62. Inserts 67 A and 67 B carry sensors 70A and 70B. Aprocess barrier 71 forms cavity 75 and prevents pressures P1 and P2 fromescaping sensor body 54 should insert 67A or 67B fail. Cavity 75 may bea vacuum or filled with an inert gas. Feed-throughs 73A, 73B and 73Cprovide electrical pathways across barrier 71 between circuit boards 57and 58.

FIG. 2 is a cut-away perspective view of insert 67A which carries sensor70A. In one embodiment, insert 70A comprises alumina. Additionally,sensor 70A should be small relative to housing 54 and positionedrelatively close to sensor 70B to reduce thermal variations and therebyimprove accuracy. This is achieved because the thermal time constant ofthe sensor is significantly less than the thermal time constant of thehousing to minimize temperature gradients within the sensing element.

FIG. 3 shows sensor 70A in accordance with one embodiment. Sensor 70Aincludes upper substrate 72 and lower substrate 74 which form cavity 76therebetween. FIG. 3 shows overall length L, thickness T, width W,minimum cavity width w of the deflecting structure of the sensing cavityand central deflection y due to applied pressure P.

Typical prior art sensors have a ratio of maximum to minimum pressurerange which can be accurately sensed of about 100:1. This is primarilylimited by non--repeatable errors in the structure and noise. Further,prior art sensors are typically limited by the use of materials withimperfect elasticity, inadequate stress isolation and poorsignal-to-noise ratio of the sensing element. For example, metal basedpressure sensors; have problems with hysteresis, material creep andrelaxation. Ceramic based sensors are formed of a matrix of crystalstypically bonded with silicon glass and also exhibit these problems.Glass-based sensors are subject to instability due to phase changes andviscosity of the glass. It has been recognized that single crystalmaterials have superior elastic properties and that sensors based onsuch materials can have improved accuracy. Single crystal diaphragmsensors have also been used, however they have typically been placedunder high tensile stress due to internal pressurization. Additionally,this type of sensor typically includes glass or metal structuralelements and uses a low strength bonding material such as glass frit,solder or epoxy. Further, this type of sensor has typically hadinadequate stress isolation.

Additionally, typical prior art sensors have used an oil fill, such assilicon oil, for use with over-pressure protection mechanisms. The oilfill is also used when coupling a corrosion resistant isolationdiaphragm to the pressure sensor. These sensors are subject to failuredue to loss of fill fluid. Typical prior art isolating diaphragms havebeen form formed in metal and are used to keep particles and corrosivematerials away from the pressure sensor. These diaphragms must be thinin order to minimize errors, however this makes the diaphragmparticularly fragile and limits life of the diaphragm. Further,different diaphragm materials are required for different applicationsand there is no metal which can be used universally.

The present disclosure sets forth a pressure sensing structure formed ofa single crystal material. Joints in the material are formed usingfusion bonding such that they are substantially free of foreignmaterials which could lead to inaccuracies. The structure may besurrounded by process fluid which applies pressure to the structure.This is possible because the structure is formed of a corrosionresistant material. The brittle material is deformed- by compressionwhich provides a higher ratio of working stress-to-error stress andtherefore a higher signal-to-noise ratio. This results because brittlematerials are stronger in compression than in tension. Thisconfiguration causes the sensor to be less sensitive to corrosion of theouter surface because the output is less dependent on the cube ofthickness and more closely linearly related to thickness. Placement ofthe structure in the process fluid improves reliability because theisolation diaphragms and oil fill are eliminated. An elongated shaftprovides stress isolation and is formed of the same single crystalmaterial to help reduce errors. Electrical leads are provided throughthe elongated shaft and isolated from process fluid. A path through theshaft can also be used to apply a reference pressure. In one embodiment,corrosion resistant material such as sapphire is used and an internalsensor is used which allows the oil fill and isolating diaphragms to beeliminated. In one embodiment, matched sensors are used as dual sensorsto measure differential pressure, which helps reduce errors common tothe two sensors. Capacitance sensing is desirable because it provides astable, low noise signal. Capacitors have no inherent thermal noise andthey have a high gauge factor with a correspondingly high output whichminimizes the noise contribution of electronic detection circuitry. Theyalso have excellent zero stability and very low zero temperaturecoefficients. These factors make it practical to detect the very lowpressure changes in a high pressure sensor that are encountered in adifferential pressure transmitter based on two independent sensingelements. Improved pressure resolution is achieved by use of electroniccircuitry.

It is well known that the deflection y due to bending in a diaphragm isproportional to ##EQU1## where w is the effective minimum width of thediaphragm and T is its thickness ##EQU2## The sensor output is thereforehighly dependent on dimensional variations.

It is known that deflection due to shear in a diaphragm proportional tow² /T. ##EQU3## This reduces the variation in output versus sensordimensions but this variation can be further reduced by relying on"bulk" deflection as defined below.

The deflection y of cavity 76 will depend on the effects of bendingdeflection, shear deflection and "bulk" deflection. Assuming that W/wequals a constant which is greater than 2, this can be expressed asfollows: ##EQU4## where: K₁ ="bulk" deflection constant for material;

K₂ =shear deflection constant for material;

K₃ =bending deflection constant for material;

w=sensor width;

P=external pressure;

y=central deflection of cavity 76 due to applied pressure P;

w=width of cavity 76;

T=thickness at slot 76 of sensor 70 (for a square cross section, T=W/2);

t=depth of cavity 76;

L=sensor length which is much greater than sensor width W and sensorthickness T;

E=Young's modulus; and

G=shear modulus.

Equation 1 illustrates that shear and bending deflection on cavity 76are dependent on cavity width w and sensor thickness T.

The term "bulk" deflection as used herein describes the K₁ P/E componentof Equation 1, where y is directly proportional to width w of cavity 76.(y∝w) Therefore, bulk deflection is highly accurate and desirable indetermining pressure and is substantially independent of variations inthickness T such as those which occur with corrosion. One aspect of thepresent invention includes providing a pressure sensor having dimensionssuch that bulk mode deflection component of total deflection of thesensor is increased.

FIG. 4 is a graph of Equation 1 which shows total deflection and itsindividual components: bulk deflection, shear deflection and bendingdeflection. When w/T is between 1 and 4, shear deflection predominatestotal deflection. As w/T gets smaller, the contribution of sheardeflection relative to bulk deflection gets smaller. For w/T less than1.0, bulk deflection is the predominant factor contributing to totaldeflection of sensor 70A due to applied pressure. Thus, one aspect ofthe invention includes pressure sensors having a ratio of w (cavitywidth) divided by T (thickness from an outer surface to an innersurface) of less than or equal to about 1.0. For shear deflection toexceed bending deflection w/T should be less than 4.0. In oneembodiment, the ratio of w/T is: 0.05<w/T<1.0. The minimum w/T value isdetermined by how small w can be practically made and inaccuracies dueto thermal gradients as T is made large.

It is desirable to minimize tensile stresses in sensors made of brittlematerials because this reduces the possibility of failure due tocracking of the sensor. One aspect of the invention includes surroundingthe sensor with the measured pressure and suitably dimensioning thesensor such that the hydrostatic compressive stress can be made toexceed the tensile bending stresses. Since the stresses are generallyadditive, the entire structure can be kept in compression. This occurswhen w/T is less than about 2.3.

Stress isolation of sensor 70A is also achieved. Stress due to mountingsensor 70A to housing 54 causes a force to be exerted on the sensor (inaddition to the force due to applied pressure) and introduces an errorinto pressure measurements. The elongated structure reduces the effectsof mounting stress to provide accurate measurement of differentialpressures and a wide operating span. Generally, mounting stressattenuates distal to the mount as sensor length L increases. Anymounting stress errors must be sufficiently attenuated over length L toachieve the desired pressure error. FIG. 4A is a graph which shows arelationship between stress attenuation and L/W for the sensor of FIG.3. The vertical axis of FIG. 4A shows the ratio of the stress at themounting point (σ_(MOUNT)) to the measured stress due to the mountingstress (σ_(MEASURED)) A change in the mounting stress (Δσ_(MOUNT))causes an error in the pressure measurement due to the change inmounting stress at the pressure sensor (Δσ_(MEASURED)). In oneembodiment, a 0.01% accuracy is required such that when measuring apressure of 4 psi the error due to mounting stress must be less than4×10⁻⁴ psi. A typical Δσ_(MOUNT) value is 400 psi such that theattenuation of the mounting stress must be 4×10⁻⁴ psi/by 400 psi=10⁻⁶.As shown in FIG. 4A, this occurs at approximately L/W of 4. In anembodiment in which two sensors are used for differential pressuremeasurement and they are matched within 10% accuracy,σ_(MEASURED/)σ_(MOUNT) is reduced by a factor 10 such that L/W isapproximately 3. In one embodiment, L/W is between 3 and 5.

FIG. 5 is a cross-sectional view showing a displacement plot for sensor70A. FIG. 5 shows one-quarter of the cross section of sensor 70A. InFIG. 5, sensor thickness T is approximately equal to cavity width w. Anapplied pressure P of 4500 psi displaces sensor 70A. In FIG. 5, abouthalf the deflection is due to shear tension and about half is due to"bulk" deflection. This is shown in FIG. 4 where shear and bulkdeflection meet. If sensor 70 were entirely in bulk mode compression,sensor 70 would retain its rectangular shape as pressure was applied.The shape distortion is due primarily to shear deflection.

FIG. 6 is a cross sectional view of sensor 120 including elongatedportion 122 and end portion 124 which form cavity 125 having width w. Inone embodiment, cavity 125 is square. Dimensions T, W and L are alsoshown in FIG. 6. End portion 124 carries capacitive plate 128 whichforms a capacitor with plate 126 carried on portion 122. Conductors 130and 132 connect to plate 126 and 128, respectively.

Pressure P causes cavity 125 to deform, thereby changing the capacitancebetween plates 126 and 128.

FIG. 7 is a cross-sectional view of pressure sensor 200. Pressure sensor200 includes top substrate 202 and lower substrate 204. Via hole 203extends through substrate 202 for coupling to electrical conductors incavity 205. FIG. 8 shows top guard conductor 210, top capacitorconductor 212, top guard conductor 214, bottom guard conductor 216,bottom capacitor conductor 218 and bottom guard conductor 220.

FIG. 9 is a top plan view of substrate 202 in which electricalconductors on the under side of substrate 202 are visible therethrough.FIG. 9 shows capacitor plate 222 connected to conductor 212 andsurrounded by guard 224 which connects to conductors 214 and 210. FIG. 9also shows vias 203 and 226 which extend through substrate 202 toconductors 210 and 212, respectively.

FIG. 10 is a top plan view of bottom substrate 204 in which electricalconductors carried on the underside of substrate 204 are visibletherethrough. In the example, substrate 204 is sapphire. FIG. 10 showscapacitor plate 228 which capacitively interacts with capacitor plate222. Plate 228 is surrounded by electrical guard 230 and temperaturesensor 232. Guard 230 shields plate 228 from stray capacitance andtemperature probe 232 changes resistance based upon temperature. Thisprovides temperature measurement of sensor 200, and allows compensationof pressure measurements based upon temperature for increased accuracy.Bonding is preferably fusion bonding, also known as direct fusionbonding, or wafer bonding in which flat, polished surfaces are mated andare bonded with the application of heat. Etching is with POCl₃ gas at900 to 1100° C. with an SiO₂ mask. It is desirable to align the crystalstructure of the substrates such that the resulting crystal structure issubstantially continuous after bonding. Further, the fusion bond shouldbe made at a temperature as close as possible to the melting point. Forthis reason, the electrode material should be capable of withstandingthe high fusion bond temperatures. For example, chrome, tungsten,tantalum, platinum and iridium allow bond temperatures in the 1300° C.to 1700° C. range so that bond strength is maximized and healing ofdiscontinuities in the crystal can occur. A typical bonding time is onehour. Other conductors include metal silicides such as molybdenumsilicide.

In a differential pressure transmitter, a temperature difference betweenthe two sensors will cause an error. Acceptable performance will requirethat the difference be less than about 1° F. and that the difference bemeasured to a precision of better than about 0.1° F. and compensated. Ina typical application, this will require that the sensor spacing be lessthan 0.5 inches.

A temperature gradient within a sensing element will also cause anerror. Acceptable performance will require that the temperaturedifference between the inside and outside of the sensor be less thanabout 0.001° F. and that the sensors be closely matched. A sensor widthor thickness of greater than about 0.25 inches will place unreasonabledemands on sensor matching in a typical application.

FIG. 11 is a simplified schematic diagram of circuitry 250 for sensingdifferential pressure using two absolute pressure sensors havingpressure responsive capacitors C₁ and C₂ carried therein. Each pressuresensor includes guard electrodes 252 which form capacitors connected toearth ground 253 of transmitter 50. The housing of the transmitter thusprovides a shield or guard to stabilize the capacitance signal andprevent electrical noise from being coupled into the circuit. Inaddition, a guard electrode can be formed on the exterior surface of thesensor or the interior surface of the ceramic inserts shown in FIG. 1.Electrical isolation can be provided to accommodate 4-20 mA circuitsthat have connections to earth ground in other places. Capacitor C₁ isdriven by square wave generator 254 and capacitor C₂ is driven by squarewave generator 256. The negative input of low noise differentialamplifier 258 is connected to the undriven plates of capacitors C₁ andC₂, and the positive input of differential amplifier 258 is connected toelectrical ground. Differential amplifier 258 has negative feedbackthrough capacitor C₁ and has charge ΔQ from capacitor C₁ and C₂ flowingin and out of the negative input. The output of differential amplifier258 is a square wave representative of differential capacitance which isconverted into a digital format by A/D converter 260. In circuit 250, ΔQis given as:

    ΔQ=V.sub.ppIN (C.sub.1 -C.sub.2)                     Equation 2

And, the amplifier output is:

    V.sub.ppOUT =ΔQ/C.sub.1 =V.sub.PPIN (C.sub.1 -C.sub.2 /C.sub.I)Equation 3

C_(I) should be selected to be approximately equal to (C₁ -C₂)/2 atmaximum differential pressure, for example, 1 pF. Additionally, tocompensate for manufacturing variations, it is desirable to haveseparate gain adjustments for each sensor.

Additionally, circuitry should be included to measure C₁, C₂ or C₁ +C₂,independently, in order to compensate for variations in output due tocommon mode or line pressure. Circuitry for detecting capacitance outputis set forth in U.S. Pat. No. 5,083,091, entitled "Charge BalancedFeedback Measurement Circuit," commonly assigned with the presentapplication.

The output from converter 260 is provided to interface circuitry 262.Interface 262 is connected to a 4-20 mA current loop and provides thedigital signal from A/D converter 260 to current loop 264 in either adigital or analog format. Interface 262 also provides power to circuitry250 from loop 264. Interface 262 is also capable of receiving commands,such as those pursuant to the HARTS communications standard.

FIGS. 12 and 13 show another embodiment having sensor body 300 whichcarries pressure sensors 200A and 200B. FIG. 12 is a sidecross-sectional view of sensor body 300 and FIG. 13 is a bottom planview of body 300. Body 300 includes circuit boards 57 and 58 connectedby wires through feed-throughs 73A, 73B and 73C through process barrier71. Sensors 200A and 200B are carried in alumina insert 302. Processbarrier 71 forms chamber 75 which may be a vacuum or filled with aninert gas. A groove 304 extends around alumina insert 302 and providesthermal and stress isolation. Mounting holes 306 are used to couple body300 to a conduit (not shown). In another embodiment, a bond between thesensor and the transmitter body is a fusion bond.

FIGS. 14 and 15 show sensor body 310 including middle shell 312 whichcarries alumina insert 314. Sensors 200A and 200B are seated in aluminainsert 314 which also carries ASIC chip 316. ASIC chip 316 performs thesame functions as circuit board 57. Groove 318 provides stress isolationfor insert 314 and sensors 200A and 200B. Process barrier 320 sealsshell 312 and provides a second barrier to process fluid. Cavity 322 inshell 312 may carry a vacuum or inert gas. Feed-throughs 324 provide apath for electric connections from ASIC chip 316 to circuit board 326.The design shown in FIG. 3 allows the sensor assembly to be tested priorto welding shell 312 into sensor body 310. High temperature operations(such as brazing the sensor to the alumina insert) can be conductedbefore mounting the sensor assembly in the housing.

In one embodiment, a pressure sensor of the invention has dimensions asfollows:

                  TABLE 1                                                         ______________________________________                                        Dimension   Size (Range)     Preferred                                        ______________________________________                                        T           0.02 to 0.2 inches                                                                             0.05 inch                                        L           0.15 to 1.5 inches                                                                             0.6 inch                                         W           0.02 to 0.2 inches                                                                             0.1 inch                                         W           0.01 to 0.1 inches                                                                             0.05 inch                                        Cavity      5 × 10.sup.-6 to 1 × 10.sup.-4                                                     0.000020 inch                                    Thickness t inches                                                            y (gap deflection)                                                                        2 × 10.sup.-6 to 5 × 10.sup.-5                                                     0.000012 inch                                    at 4500 psi inches                                                            ______________________________________                                    

A typical capacitance for the sensor is 42 pF at zero psi.

In one embodiment, parameters of a pressure sensor are as follows:

                  TABLE 2                                                         ______________________________________                                                    ZERO      FULL SCALE SPAN                                         PROPERTY    (0 psi)   (4500 psi) (4500 psi)                                   ______________________________________                                        Gap (At Center)                                                                           20        11.25      8.75                                         (μ Inches)                                                                 Gap (Effective                                                                            20        12.37      7.63                                         Average)                                                                      (μ Inches)                                                                 Sensor      42.2      68.2       26.0                                         Capacitance                                                                   (No Parasitics)                                                               (pf)                                                                          Parasitic   1.97      1.994      0.024                                        Capacitor                                                                     (w/o Guard                                                                    Shield and                                                                    assumes 0.6                                                                   inch length)                                                                  (pf)                                                                          Sensor                                                                        Temperature                                                                   Coefficient                                                                   (PSI/C)     -0.059    0.534      0.593                                        [ppm/C]     -13.1     118.6      131.7                                        ______________________________________                                    

FIGS. 16A through 16G show cross-sections of a pressure sensor inaccordance with one aspect of the invention. FIG. 16A shows arectangular structure in which all interior angles are 90°. FIG. 16Bshows a hexagonal sensor structure in which all interior angles are 60°.A rhombus structure is shown in FIG. 16C in which two inner angles are60° and two angles are 30°. FIG. 16D shows a triangular structure inwhich all interior angles are 60°. The structures shown in FIG. 16Athrough 16D are convenient structures for sapphire because they tend tolie along planes of a sapphire crystal. FIG. 16E shows a sensor 340 inwhich a rounded portion 342 is coupled to a rectangular portion 344.Cavity 346 is formed in rectangular portion 344. FIG. 16F shows anotherembodiment in which the cavity is formed in the rounded portion. Variousalternatives are also available including rounding both portions, suchas shown in FIG. 16G. A round cross section is desirable because it canbe closely fit into a round hole and can be sealed with a round O-ring.It can be fabricated by machining a square structure with a diamondgrinding wheel.

FIGS. 17A through 17B show example configurations of the electrodes forcapacitive plates. Electrode instability causes errors in capacitivepressure sensors which have small gaps. Residual stress causes thepressure sensor structure to warp. Further, dimensional changes in theelectrode surface change the gap dimension t. These changes can becaused by oxidation and reduction or migration of atoms on the opposingsurfaces. FIGS. 17A and 17B show examples of a solution to this problemin which the capacitor plates are broken apart into strips having widthsand spacings which result in a capacitance that is substantially equalto a solid electrode. For example, if the substrate has a dielectricconstant of 10 times the gap dielectric constant, then the spacingscould be about 10 times the gap dimension and the widths could be lessthan about the gap dimension. This reduces the amount of materialavailable to warp the sensor structure. Further, the strips can beconfigured so that most of the electrical flux emerges from the backside of the electrodes. As the back side of the electrode is in contactwith sapphire, it is protected from surface effects and will provide astable capacitance even if the inside surface changes dimensions. FIGS.17A and 17B show two example configurations which increase the amount offlux emanating from the back side of the electrodes. Variations on thisembodiment include providing guard electrodes (298) which overlie eachplate electrode and shield each plate electrode. Although these platesare spaced apart, they are similar to a continuous capacitor platebecause of their spacing and relative size.

In one embodiment, the capacitor electrodes are implanted into thesurface of the substrate. This provides electrode stability byprotecting the electrode and helping to reduce the amount of the changein the capacitance over time. A sapphire substrate is implanted with Vions at a dose of 1×10¹⁸ ions/cm² and an energy level of 200 KeV. Thischanges the resistivity of the sapphire from very highly resistant toabout 15 Ohms/sq. The implanting process concentrates most of the Vapproximately 1000 Å beneath the original sapphire surface. An exampleof an implanted electrode 300 is shown in phantom in FIG. 17B.

FIG. 18 is a graph of bonding temperature as a percent of melting pointtemperature versus bond strength as a percent of material strength. Formaximum stability and accuracy, it is desirable to have the bondstrength as close as possible to the material strength so that thesensor structure behaves as a unitary body. In one embodiment of theinvention, bonding temperature is the range indicated at 350 in FIG. 18.Sapphire, quartz or silicon can be used in forming a pressure sensor,and their melting points are 2050° C., 1723° C. and 1415° C.,respectively. The desired range 350 shown in FIG. 18 is from atemperature of about 68% of the melting point absolute temperature. Inone embodiment, bonding temperature should not exceed about 95% of-themelting point temperature. The desired fusion bond as used herein is abond having a strength which is substantially the same as the strengthof the underlying crystal material, and is formed by the application ofheat with substantially no foreign material in the bond.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention. Single crystal materials include sapphire,silicon, quartz, ruby and diamond. Generally, these are materials withlow hysteresis which are highly stable thereby providing little creep.In general, materials with higher melting points will be more stablethus, sapphire is one preferred material. The cavity of the sensor canbe a vacuum or filled with a fluid or gas. The electrodes can includeconductors such as metals, metal oxides or doped semiconductors and canbe protected with an insulating dielectric material such as quartz. Inanother embodiment, the stress isolation structure is formed of apolycrystalline material. Further, the bonding can be formed with orwithout the use of pressure.

What is claimed is:
 1. A pressure transmitter for use in an industrialprocess control system for transmitting pressure on a process controlloop, the transmitter comprising:I/O circuitry coupling to the loop fortransmitting information on the loop; compensation circuitry receiving apressure related signal and responsively controlling the I/O circuitryto transmit pressure information on the loop; a transmitter housingcontaining the I/O circuitry and the compensation circuitry; a mountingportion in the housing formed of a mounting portion material; a processbarrier in the housing positioned between the I/O circuitry and thecompensation circuitry which isolates the I/O circuitry from thecompensation circuitry and the mounting portion; an elongated memberformed substantially of a brittle, corrosion-resistant material which isdifferent from the mounting portion material and is coupled to themounting portion at a proximal end and having a passageway extendingtherethrough; a pressure responsive portion at a distal end of theelongated member for immersion in a fluid which is at a pressure relatedto the process fluid pressure and deforming in response to process fluidpressure; and a sensor coupled to the pressure responsive portion andhaving an electrical connection to the compensation circuitry throughthe passageway of the elongated portion and a sealed feed-through in theprocess barrier, wherein the sensor and electrical connection areisolated from process fluid by the elongated member; wherein theelongated member and pressure responsive portion include fusion bondswhich are substantially free of foreign material.
 2. The pressuretransmitter of claim 1 wherein the elongated member defines a cavitytherethrough for applying a reference pressure to the pressureresponsive portion.
 3. The pressure transmitter of claim 1 including asecond elongated member coupled to the mounting portion and carrying asecond pressure responsive portion having a second sensor coupled to thecompensation circuitry, wherein the pressure information relates to adifferential pressure.
 4. The pressure transmitter of claim 1 whereinthe elongated member and the pressure responsive portion have a width wand a thickness T, and w/T is less than about 4.0.
 5. The pressuretransmitter of claim 4 wherein the elongated member and the pressureresponsive portion have a width w and a thickness T, and w/T is lessthan about 2.3.
 6. The pressure transmitter of claim 5 wherein theelongated member and the pressure responsive portion have a width w anda thickness T, and w/T is less than about 1.0.
 7. The pressuretransmitter of claim 1 including electrical conductors implanted in theelongated member and providing the electrical connection between thesensor and the compensation circuitry.
 8. The pressure transmitter ofclaim 1 wherein the elongated member has a length L exposed to processfluid and a maximum width W and L/W is greater than about 3.0.
 9. Apressure transmitter for use in an industrial process control system fortransmitting a process variable related to pressure on a process controlloop, comprising:an elongated stress isolated pressure responsivestructure of a brittle, corrosion resistant material comprising a firstsubstrate of single crystal material bonded to a second substrate ofsingle crystal material and having a passageway extending through apressure responsive portion to a distal end; a capacitance sensorcoupled to the pressure responsive portion of the pressure responsivestructure providing a capacitive output in response to deformation ofthe pressure responsive portion due to an applied pressure from aprocess fluid; and transmitter circuitry coupled to the sensor adaptedto measure the process variable related to the applied pressure andresponsively transmit the process variable on the process control loop;wherein the bond between the substrates is a fusion bond which issubstantially free of foreign material and has a bond strengthsubstantially equal to the single crystal material strength and isformed at a temperature between about 65 percent and 95 percent of anabsolute temperature corresponding to a melting point of the material.10. The pressure transmitter of claim 9 wherein the pressure responsivestructure has a cavity formed therein with a width w and a thickness T,and w/T is less than about 4.0.
 11. The pressure transmitter of claim 9wherein the passageway provides a reference pressure proximate thecapacitance sensor.
 12. The pressure transmitter of claim 9 wherein thepressure responsive structure includes electrical conductors extendingtherethrough able to withstand a temperature used to form the bond. 13.The pressure transmitter of claim 12 wherein the electrical conductorsare implanted in the pressure responsive structure.
 14. The pressuretransmitter of claim 12 wherein the material comprises sapphire.
 15. Apressure transmitter for use in an industrial process control system fortransmitting a process variable related to pressure of a process fluidon a process control loop, comprising:an elongated pressure responsivestructure of a single crystal, corrosion-resistant material, theelongated structure having a width w and a thickness T for coupling to aprocess fluid; a capacitance sensor coupled to a distal end of thepressure responsive structure providing a capacitive output in responseto deformation of the distal end of the pressure responsive structuredue to an applied pressure from a process fluid; and transmittercircuitry coupled to the sensor adapted to measure the process variablerelated to the applied pressure and responsively transmit the processvariable on the process control loop; wherein w/T is less than about 4.0and the elongated pressure responsive structure is bonded together byfusion bonds which are substantially free of foreign material.
 16. Thepressure transmitter of claim 15 wherein the pressure responsivestructure includes more than one substrate and a bond between substratesis a fusion bond which has a strength substantially equal to the singlecrystal material strength and is formed at a bonding temperature betweenabout 65 percent and about 95 percent of an absolute temperaturecorresponding to a melting point of the material.
 17. The pressuretransmitter of claim 16 including electrical conductors in the pressureresponsive structure having a melting point greater than the bondingtemperature.
 18. The pressure transmitter of claim 15 wherein theelongated pressure responsive structure includes a cavity extendingtherethrough for providing a reference pressure proximate thecapacitance sensor.
 19. The pressure transmitter of claim 15 includingelectrical conductors implanted in the pressure responsive structure.20. The pressure transmitter of claim 15 wherein th pressure responsivestructure is coupled to a pressure transmitter housing by a stressisolating structure of the same single crystal, corrosion-resistantmaterial.
 21. The pressure transmitter of claim 20 wherein the pressureresponsive structure and stress isolating structure are surrounded byand directly exposed to process pressure fluid.
 22. The pressuretransmitter of claim 15 wherein a joint between the pressure responsivestructure and the pressure transmitter housing is a fusion bond.
 23. Thepressure transmitter of claim 15 wherein the capacitance sensor includesconducting electrodes formed of conducting strips on opposing surfacesof a cavity gap.
 24. The pressure transmitter of claim 23 wherein theconducting strips have width and spacing substantially equal or lessthan a cavity gap spacing.
 25. The pressure transmitter of claim 15wherein the material comprises sapphire.
 26. A transmitter for use in anindustrial process control system for providing an output related to adifferential pressure of a process fluid, comprising:a transmitter body;a first pressure responsive structure formed substantially of a singlecrystal material coupled to a first process pressure; a first sensingelement coupled to the first pressure responsive structure having anoutput related to the response of the first pressure responsivestructure to the first process pressure; a first elongated stressisolation member extending between and coupling the first pressureresponsive structure to the transmitter body, the first stress isolationmember formed substantially of the single crystal material; a secondpressure responsive structure formed substantially of a single crystalmaterial coupled to a second process pressure; a second sensing elementcoupled to the second pressure responsive structure having an outputrelated to the response of the second pressure responsive structure tothe second process pressure; a second elongated stress isolation memberextending between and coupling the second pressure responsive structureto the transmitter body, the second stress isolation member formedsubstantially of the single crystal material; and output circuitry inthe transmitter body coupled to the first and second sensing elementsfor determining the difference in pressure between the first and secondprocess pressures and providing an output related to a differentialpressure therebetween; wherein the pressure responsive structures andthe elongated stress isolation members included are formed by substratescoupled together by fusion bonds which are substantially free fromforeign material.
 27. The transmitter of claim 26 wherein the first andsecond pressure responsive members are exposed directly to the processfluid and the single crystal material is substantially resistant tocorrosion due to the process fluid.
 28. The transmitter of claim 26wherein the first and second sensing elements comprise capacitorelectrodes and the output is a capacitance related to pressure.
 29. Thetransmitter of claim 26 wherein the first and second pressure responsivestructures are coupled to first and second reference pressures and thepressure sensing structures are responsive to a difference between areference pressure and a process pressure.
 30. The transmitter of claim24 wherein the reference pressures are coupled together and are equal.31. The transmitter of claim 30 wherein the reference pressures aresubstantially independent of process pressure.
 32. The transmitter ofclaim 29 wherein the reference pressure is substantially a vacuum. 33.The transmitter of claim 29 wherein the reference pressure istransmitted via a gas.
 34. The transmitter of claim 26 wherein thesingle crystal material is sapphire.
 35. The transmitter of claim 26wherein the first and second pressure responsive structures aremaintained at substantially the same temperature by the transmitterhousing.
 36. The transmitter of claim 35 wherein the pressure responsivestructures are spaced apart such that they maintain a temperaturedifferential within a predetermined range over a predeterminedtemperature gradient in the transmitter housing.
 37. The transmitter ofclaim 26 wherein a process pressure range is greater than 1000 psi and adifferential pressure range is less than 30 psi.
 38. The transmitter ofclaim 26 wherein each pressure responsive structure is maintained at asubstantially uniform temperature throughout.